The Greatest Show On Earth The Wildebeest Migration The annual movement of wildebeest and other grazing herbivores, notably zebra, across the Serengeti-Mara ecosystem is one of the greatest spectacles in the natural world. Over two million herbivores partake in this great migration, with some 200 000 zebra ahead of, and 500 000 Thomson's Gazelle behind, the main players - one and a half million wildebeest. Not surprisingly, this impressive phenomenon is determined by the availability of grazing - which is dependent upon rainfall - but there are several other factors that shape this seasonal movement.

Essentially, the wildebeest are taking advantage of the strongly seasonal conditions, spending the wet season on the plains in the south-east, and the dry season in the woodlands of the north-west, but the sheer weight of their numbers means that they themselves play a role in shaping their environment to their needs. Members of the vast wildebeest herd give birth more or less simultaneously - usually over a period of three weeks sometime between January and March - when optimum grazing is available on the short grass plains at the base of the Gol Mountains. By occupying and birthing on these open plains, the mass of herbivores reduces their competitive pressure on other grazers in the Serengeti ecosystem for up to half of the year, while also avoiding the muddy and fly-infested woodlands during the wet season. The short grass plains are green only during the rains and the mostly treeless landscape provides optimum conditions for the females to give birth as potential predators are more easily detected here than in the woodlands. Unlike most other antelope, wildebeest do not hide their young, but encourage them to get on their feet and join the herds where there is safety in numbers.

The migration is rarely ever the same in terms of precise timing and direction, as local conditions influence grass growth. So it is, that the wildebeest may move off the open plains earlier in some years, and remain in the northern woodlands for longer, in others. Fire in an area may render it temporarily unsuitable for herbivores due to an absence of grass fodder, but such areas soon flush with nutritious new growth and are then a major attraction.

Perhaps the most interesting thing about the great migration is the way in which the wildebeest themselves have shaped - and continue to regulate - the structure of the ecosystems upon which they are dependent. The composition of grass species is influenced by grazing, so with more than one and half million hungry mouths on the plains at one time, those grass species which are able to tolerate the pressure will out compete those that cannot. Indeed, some grasses only flourish when constantly grazed. The timing of the wildebeest calving is probably linked to the timing of the rut at the end of the rains in May and June. The wildebeest move off the plains at this time, even when sufficient grazing remains, in order that they will all be compressed into a smaller area which is necessary to synchronize the rut. Interestingly, the rut itself appears to coincide with the full moon suggesting that the mating peak is triggered by the lunar cycle (most mating takes place between full moons). Migration may reduce predation on wildebeest, in that their chief predators - lion and hyena - are limited in their own movements by their need to provision food to non-mobile young. Certainly, predators have no significant impact on herbivore numbers in the Serengeti, whereas the non-migratory wildebeest of Ngorongoro are limited in number by the resident hyena and lion. For the migrants, there is, however, a high mortality rate due to injury and perhaps fatigue, so large numbers of griffon vultures (whose own breeding cycle is coordinated accordingly) following the herds on their journey.

Typically, the wildebeest head north-west from the short grass plains to the Western Corridor of the Serengeti and its Grumeti River. This watercourse is their first real obstacle and gigantic crocodiles are waiting for the hesitant wildebeest to stumble at the crossing. From Grumeti, the herds move north - often spilling over into the Klein's Camp Concession - before crossing the Kenyan border into the Masai Mara. Here again, they must cross a river, this time the Mara with its flotillas of hungry crocodiles. The mass of grunting gnus remain on the productive Mara grasslands until October or November at a time when the rest of the ecosystem is simply too dry to support them. And then, as the storm clouds gather in the south, the vast herds return to their breeding grounds which, by the time they arrive, are once again green and lush.